1. Field of the Invention
The present invention relates to photopolymerizable compositions, and more particularly to photopolymerizable compositions containing photopolymerization initiators.
2. Description of the Prior Art
It is known that upon irradiation of compositions such as printing inks, adhesives, etc., in which unsaturated compounds capable of being hardened upon irradiation with electromagnetic radiation are employed as components thereof, with electromagnetic waves such as visible light, ultraviolet light, X-rays, etc., or radiation such as electron beams, neutron beams, .alpha.-rays, etc. that the above-mentioned compounds in the compositions are polymerized and thereby hardened. Further, it is known that the polymerization rate increases remarkably if such irradiation is performed in the presence of photopolymerization initiator compositions. Such techniques are described in, for example, U.S. Pat. Nos. 3,551,235, 3,551,246, 3,551,246, 3,551,311 and 3,558,387, Belgian Patent 808,179, Japanese Patent Application (OPI) No. 110781/75 (corresponding to British Pat. No. 1,467,645). These compositions obtained using such techniques are characterized as possessing excellent properties such as bending property, resistance to chemicals, anti-abrasion, luster, adhesive property, color hue, etc. However, on the other hand, these compositions have the disadvantages that since the sensitivity of the compositions to hardening is poor and a long period of time is required for imagewise exposure upon formation of images, images of good quality are not produced in the case of elaborate images if there is a slight vibration in operation; and since the amount of energy emitted from a light source or a radiation source for exposure should be increased, release of a vast amount of heat accompanied thereby must be taken into consideration and further deformation as well as change in the quality of films of the composition tend to occur due to the heat, and the like.
After much investigation to eliminate the above mentioned disadvantages by increasing the sensitivity of photopolymerizable compositions, it has now been found that a photopolymerization initiator having a specific composition markedly increases the photopolymerization rate of a photopolymerizable compound having an ethylenically unsaturated double bond therein, and the present invention has been accomplished.